Chickens with 5 different phenotypes for the expression of endogenous avian C-type virus (K16 chf+gs+, K18 chf+gs-, line 7 base 2 chf-gs- inducible for subgroup E virus, line 15B chf-gs- inducible for an uncharacterized avian C-type virus, and K(-) chf-gs-) will be used in breeding experiments to determine whether these phenotypes are inherited from linked or unlinked genes. The expression of the different phenotypes will be examined in heterozygotes to determine the dominance and look for possible interactions of the various genotypes. The horizontal spread of endogenous avian C-type viruses will be examined with particular emphasis being placed on characterizing the growth of subgroup E viruses in chicken cells. The intracellular mechanism which appears to restrict the growth of subgroup E viruses in some subgroup E virus susceptible chicken cells (K28) but not others (line 15) will be examined for its inheritance and mode of action.